The present invention relates generally to passive determination of terrain parameters in relation to a vehicle. More particularly the invention relates to a method of determining a terrain parameter according to the preamble of claim 1 and an on-board arrangement according to the preamble of claim 10. The invention also relates to a computer program according to claim 7, a computer readable medium according to claim 8 and an air vehicle according to claim 16.
Today, there exist many alternative means of navigation aids for vehicles, such as aircrafts. For instance, in the last decades, global navigation satellite systems (GNSS:s) have been developed which render it possible to determine a vehicle's position on the basis of accurate radio signals received from a plurality of satellites (at least four). The most wide spread GNSS standards are represented by the U.S. GPS (Global Positioning System), the Russian correspondence GLONASS (Global Navigation Satellite System) and the European Galileo satellite system. In military applications, however, it is generally not advisable to rely on an external system such as a GNSS, since this type of system may be struck out or its signals may be spoofed and/or distorted.
Instead, a purely local solution is normally used, such as a radar altimeter or a sonar in combination with a terrain database. Nevertheless, passive systems, i.e. where no signals are emitted, are preferable because this strategy reduces the risk of detection. Additionally, a passive system is less sensitive to distortion and spoofing by others.
For instance, passive ranging may be accomplished by means of triangulation, wherein angular sensors are used to measure angles to a remote point in the terrain. Teodolites and cameras may here function as angular sensors. Basically two different strategies may be applied—stereo measurement by means of multiple sensors, or time separated measurements where a single sensor is placed at different locations, however aimed against the same point.
Stereo measurement is advantageous because it enables distance determination based on a single image, so that an essentially instantaneous result may be obtained. However, in order to attain a high accuracy, the distance between the angular sensors must be relatively large. Thus, the vehicle's dimensions must be comparatively large. Normally, at least in aircraft applications, this is not the case. Therefore, stereo measurement is an unsuitable technology for determining the distance between an aircraft and the ground.
However, by performing the measurements at different time instances a sufficiently large sensor separation may be achieved. Specifically, a first image is recorded at a first point in space, and a second image is recorded at a second point in space. Provided that the spatial distance is known between the first and second points and that a number of identifiable terrain points occur in both images, it is possible to calculate the distance to a remote point with high accuracy.
One important factor, which may severely influence the accuracy is that the angular sensor may be oriented differently when recording the second image than when recording the first image. Typically, such a change in orientation is due to that the vehicle has altered its attitude angle and/or rolled between the first and second points in space. Of course, this problem is particularly accentuated in aircraft applications. Consequently, in addition to the spatial distance (or translation) between the image registration points, the attitude change (or rotation) must be known.
The U.S. Pat. Nos. 5,373,318 and 5,367,333 describe passive range measurement systems, wherein optical flow data is used to determine distances to various objects in the terrain. Both systems, however, require input data in the form of a so-called drift angle which describes the total change of the attitude angle due to a rotation of the camera and/or the vehicle onto which the camera is mounted. Although today's inertial sensors (e.g. including gyros, microactuators, and/or microsensors) may provide very accurate translation data, it is still problematic to register the relevant rotation with sufficiently high accuracy. Consequently, the calculated range is not a reliable figure either.